Conventionally, the seal ring of this type is used in, for example, a hydraulic apparatus such as an automatic transmission for an automotive vehicle or the like.
A conventional seal ring will now be described below with reference to FIGS. 6 to 9. FIG. 6 is a schematic plan view of the conventional seal ring and FIG. 7 is a schematic cross-sectional view showing a state where the conventional seal ring is mounted.
Also, FIG. 8 is a perspective view showing a state of a cut portion (special step cut) of the conventional seal ring.
The seal ring 100 shown in the figures is adapted to seal an annular gap between a housing 200 provided with an axial hole and a shaft 300 inserted into this axial hole, and is used through being mounted on an annular groove 301 formed in the shaft 300.
The seal ring 100 is made of a resin material and is provided with a second seal portion 101 for sealing a side wall surface of the annular groove 301 provided in the shaft 300 and a first seal portion 102 for sealing an inner circumferential surface provided in the housing 200.
Then, when a pressure is applied on in a direction indicated by an arrow P toward a non-sealed fluid side A from a sealed fluid side O, the seal ring 100 is depressed on the non-sealed fluid side A, and thus the second seal portion 101 depresses the side wall surface of the annular groove 301, and, on the other hand, the first seal portion 102 depresses the inner circumferential surface of the axial hole provided in the housing 200 facing the annular groove 301, to thereby perform sealing in the respective positions.
In this manner, leakage of the sealed fluid to the non-sealed fluid side A is prevented.
Here, the sealed fluid means, for example, a lubricating oil, particularly such as an ATF in a case where it is used for an automatic transmission for an automotive vehicle.
Also, as shown in FIG. 6, a cut portion S0 is provided at one position in a circumferential direction in a ring body of the seal ring 100 for the purpose of enhancing assembling property or the like.
Various kinds of forms are known as such a cut portion S0. However, as a cut portion that may preferably cope with an issue of ambient temperature change, a special step cut having a shape cut into a two-step form as shown in FIG. 8 is known.
In this special step cut, since the sealed fluid side and the non-sealed fluid side are interrupted from each other while each vertical surface to the circumferential direction has a gap T in the circumferential direction, even if the ring body is expanded due to heat, an amount of change in dimension may be absorbed for such amounts that correspond to the gap T while keeping the sealed condition. Thus, it is possible to keep the sealing performance even against the ambient temperature change.
In the seal ring 100 described above, particularly in a case where the shaft 300 is made of a soft material such as an aluminum alloy, the second seal portion 101 and the side wall surface of the annular groove 301 are worn, respectively, due to the friction between both.
This is because a lubricating film of the lubricating oil is hardly formed between the second seal portion 101 and the side wall surface of the annular groove 301. In particular, in a case where foreign matters existing in the lubricating oil are entrained therebetween, the frictional wear is remarkable.
As a technical countermeasure for reducing such a frictional wear, there is known a technique to change the contact relationship between the second seal portion 101 and the side wall surface of the annular groove 301 from a conventional area contact to a linear contact to thereby facilitate the formation of the lubricating film so as to enhance wear resistance property.
FIG. 9 shows schematic views showing a seal ring in which the contact relationship is the linear contact. FIG. 9(a) is a schematic cross-sectional view, and FIG. 9(b) is a perspective view of an outline of a cut portion (special step cut).
Namely, as shown in FIG. 9, a seal ring 110 having a tapered cross-sectional shape is used, and a taper portion 111 of the seal ring 110 is brought into contact with a corner portion 302 that is formed by the side wall surface of the annular groove 301 and the outer circumferential surface of the shaft 300 so that the contact condition between the seal ring and the side wall surface is made linear, to thereby form the second seal portion to facilitate the formation of the lubricating film and enhance the wear resistance property.
However, there are cases where the above-described conventional technology generates the following problems.
In the above-described seal ring, as shown in FIG. 9(b), the corner portion 302 formed by the side wall surface of the annular groove 301 and the outer circumferential surface of the shaft 300 is brought into contact with the gap T of the taper portion 111 of the seal ring 110.
The gap T absorbs change in a circumferential length of the ring body in response to the ambient temperature change to keep the sealing performance, and its circumferential length is changed in accordance with the ambient temperature change.
Accordingly, in a case where the gap T becomes an oil leakage path indicated by an arrow in the drawing and, in particular, in a case where the circumferential length of the ring body is shortened due to the temperature change, the oil leakage is increased.
The present invention was made in view of solving the above-mentioned problem in the conventional technology, and an object of the invention is to provide a seal ring that is superior in quality for keeping stable sealing performance for a long period of time.